The invention is based on a fuel injection pump for internal combustion engines.
In a fuel injection pump of this type, known from British Patent No. 893,621, a fuel line discharges into the pump cylinder via an intake bore that is opened at bottom dead center by the face end of the pump piston, by way of which fuel can flow into the pump work chamber before the onset of the delivery stroke. An outlet opening also branches off from the pump cylinder and is opened by the first, helically extending control edge on the pump piston in order to terminate the effective pump piston delivery stroke. The first control edge on the pump piston defines a circumferential groove provided there, which is in continuous communication with the pump work chamber via a longitudinal groove. Additionally provided in the jacket face of the pump piston is a circumferentially extending recess, which can be made to coincide with the recess provided in the jacket of the pump cylinder; the latter recess, after a beginning stroke of the pump piston once its end edge has closed the inlet opening, still communicates with the outlet opening but does not yet communicate with the recess in the jacket face of the pump cylinder. After an initial delivery stroke, the recess on the pump piston comes to communicate with the recess in the cylinder jacket face, while simultaneously communicating with the outlet opening. Furthermore, the recess in the pump cylinder jacket also communicates with the pump work chamber, but after a stroke of the pump piston is closed by the piston's end edge. At that point, the recess in the pump piston still communicates with the outlet opening. The effect of this embodiment is that over the portion of the pump piston stroke via which communication between the outlet opening and the pump work chamber is established via the recess in the pump piston and the recess in the pump cylinder jacket, the fuel injection is interrupted, and it is not continued, as a main injection, until after the closure toward the pump work chamber of the recess in the pump cylinder jacket, when the outlet opening is opened by the first control edge.
Fuel injection pumps operating by the same principle are known for instance from German Patent Disclosure Documents DE-OS 38 09 700, 29 22 426 and 37 31 817. With such embodiments, a small preinjection quantity is supposed to be injected into the engine cylinders first, in the course of the pump piston delivery stroke, with the main injection quantity injected only afterward, after an interruption in the injection. The goal of these embodiments is by the selected disposition of recesses to attain a very brief interruption in fuel delivery, which is shorter than if, for example, by means of a transverse groove in the pump piston jacket face communicating continuously with the pump work chamber, a typical relief bore were opened over its entire diameter adjoining the control edge determining the delivery. The preinjection a short interval ahead of the main injection is required in order to reduce noise of engines operated with direct injection. In a known manner, during the ignition delay in continuous injection at a high injection rate, relatively large quantities of fuel are stored ahead of time before ignition begins. The pre-stored quantity of fuel that then combusts suddenly produces a steep pressure rise, which makes itself felt in the form of noise. To avoid the development of black smoke during combustion or to reduce it, and to keep fuel consumption low, a correct graduation of the preinjection quantity and of the interval between the preinjection and the main injection must be adhered to. The conditions for this vary fundamentally as a function of load and rpm, but nevertheless a variable control of the preinjection quantity and the preinjection interval would entail very considerable effort and expense. It is therefore a goal to embody the construction of fuel injection pumps in such a way that a reasonable combustion behavior can be obtained with a single set of injection equipment, with a structurally specified preinjection fuel quantity and interval between preinjection and main injection. If the intervals between coincidence of the recesses remain constant, an rpm dependency results from rpm-dependent predelivery and postdelivery effects. Because of the throttle influence and the fact that the coincidence becomes shorter and shorter in terms of time with increasing rpm, the preinjection quantity increases over the rpm. Yet the requirement of the engine for optimal conditions is precisely the opposite.